Climate/ environment and Rift Valley Fever

RVF cycle in Kenya
Prehaud and al, 1997 from Linthicum and al., 1986

Incidence of climate and environment on human health is well-known. Local climate conditions have biological implications on some diseases vectors and can modulate the Extrinsic Incubation Period (EIP) of vectors as well as behavior of pathogenic agents.

Satellites monitoring from space agencies such as
CNES, CONAE, and NASA among others, with derived added-values products and analyses, can help providing real-time climate and environment variability possibly linked to epidemics’ risks.

In Kenya, Linthicum and al. (1999) showed that outbreaks of RVF followed periods of abnormally high rainfall. Even if rainfall might not be the only confounding factor for RVF outbreaks everywhere , in the latter case rainfall increase in Kenya is linked with negative Southern Oscillation Index (SOI) anomalies and warmer equatorial Indian Ocean.

Southern Oscillation Index anomalies and RVF activity
A time series plot of SOI anomalies between January 1950 and May 1998. Periods of RVF activity in Kenya are depicted. Monthly SOI values are shown as standardized deviation based on the 1951-80 mean. (Linthicum and al., 1999)

In semi-arid and arid areas such as the Sahel, things might be different to what happen elsewhere (see Lefevre, 1997 ; Lacaux and al., 2005). The Senegal climate is defined by both its geography and aerological factors. Thus its tropical climate component varies from the coastline eastward with a monotonous relief facilitating three basic atmospheric circulation (deflected tradewinds, harmattan and summer monsoon) (Sagna, 2000).

Seasonal contrasts are associated with the position of the
InterTropical Convergence Zone (ITCZ)
over the Easten Atlantic Ocean and West Africa. Rainfall is characterized by its spatio-temporal variability. Most of rainfall amount (more than 80%) comes from deep convection and squall-lines.
Generally, the dry season lasts 6 to 9 months from south-to-north and the rainy season which generally goes from June to October. The annual rainfall amount display a meridional gradient over Senegal. Sagna (2000) was able to distinguish five climatic zones in Senegal (seesFigure).

The Senegal river basin is where the RVF epidemics belongs to two climatic zones : the ‘stable’ maritime wind zone (i.e., Saint-Louis area) and the Sahelian zone (including Podor, Matam and Bakel).
Contrary to what happened in Kenya, the Rosso epidemics in 1987, was not associated with heavy rainfall (see Ndione and al., 2003 ; Ndione and al. 2005 ; Mondet and al., 2005). In addition, the RVF virus circulation over southeastern Mauritania was detected during the extreme drought period from 1982 to 1985 (Saluzzo and al., 1987 ; Zeller and al., 1997 ; Lefevre, 1997).
Such events would imply that environment/climate conditions and epidemiological mechanisms leading to RVF epizootics may differ from those detected over East Africa ([Lefevre, 1997 ; Ndione and al., 2003->66) even if rainfall events might influence RVF emergence. Considering RVF events in Senegal, we might consider rainfall intra-seasonal variability and its spatio-temporal distribution to explain amplification of vectors (i.e., mosquitoes) and therefore RVF viral diffusion (Ndione and al., 2003 ; Ndione and al., 2005 ; Lacaux and al., 2005 ; Mondet and al., 2005).